Venting a plastic covered hoop house

I have a very low, PVC/rebar and plastic sheet greenhouse. I need to attach a vent with an automatic opener. Do I need two vents? Or will one high up work? The tunnel is 12 feet wide and 20 feet long. How many square feet of vent would I need?

I'm just now about to order the plastic for my pvc greenhouse here in Arkansas. I'm putting one vent up high and another low on opposite end, which a fan in from of it. I figure just one will let hot air out but not as fast as I want it out. I'll have two automatic vents and a temp controlled fan. How did yours work out?

What does a metric clock look like? I bet it is nothing like this tiny ad: